Primary roof and rib control of underground openings is traditionally accomplished by metallic supports that may have a variety of configurations. Perhaps the most common are steel rock bolts which are generally spaced on a regular pattern within the roof, and sometimes sidewalls (ribs) of the openings. These primary support devices are intended to prevent catastrophic failure of the openings; they are generally not suited to prevent localized failure (i.e. debris falls or spalls) between their isolated locations. Herein lies the application of supplemental (or secondary) roof and rib control (support).
Underground mines and tunnels presently employ steel products, such as welded wire mesh, chain-link fence and/or expanded metal screen for supplemental support of their openings. These steel products are heavy and difficult to handle. Further, their edges are very sharp and are frequently the cause of mine or tunnel worker injuries. The steel products also impede extraction of the ore body (e.g. coal seam), since they are damaging to the cutting equipment used to extract the ore and/or soil and rock materials. Moreover, while steel products are resistant to combustion, they are subject to corrosion in the damp underground environments.
An example of a steel roof support is disclosed in U.S. Pat. No. 4,003,208 to Hornung et al. This patent discloses a shield support structure for preventing the falling of dust and debris from the roof and from the break of a coal mine. The assembly includes roof bars and a roof covering disposed between the roof of the mine and the roof bars. The roof covering includes a welded wire netting sheet having longitudinal and transverse wires with a dust-tight web welded or fixed in between the longitudinal and transverse wires.
Another example of a steel support of a roof of a coal mine is disclosed in U.S. Pat. No. 4,245,926 to Asszonyi et al. which discloses a welded grid structure for securing underground cavities. The welded grid structure includes two sets of grid systems of parallel straight bars disposed in parallel planes and connected by welding to a third set of bars that extend perpendicular to the first two set of bars.
An example of equipment for laying out a layer of elongate material made of wire mesh is disclosed in U.S. Pat. No. 4,230,371 to Bell et al. In this patent, equipment is disclosed for laying a layer of wire mesh adjacent to an exposed mine roof on the body of a mining machine, the equipment being cantilevered from the body of the machine into the newly formed track left directly behind the cutter.
Polymer grid products have previously been used to control roof caving during the recovery of shield supports in longwall mining of coal. The application of polymer grids was supplemental to primary roof control devices of rock bolts and cables, wire ropes, or threaded rebars. However, the term of use was temporary; that is, longwall shield recovery requires roof control serviceability for a period of no more than several days. Because the flammability hazard is usually very brief, the polymer grid products used were neither flame-retardant nor self-extinguishing.
This short-lived application of supplemental roof control is distinguished from the long-term application of supplemental roof and rib support of the instant invention wherein a permanent safeguard against materials which would otherwise contribute to hazards of fire or even explosion is necessary.
Indeed, the presence of metallic supporting material exacerbates the hazard because a spark can initiate an explosion and cause a fire. While replacement of metallic materials with polymeric materials minimizes the danger of sparking, non-flame-retardant and/or self-extinguishing polymer grid materials may sustain and spread a fire that has other origins. Thus, it is primarily the provision of a fire-retardant and/or self-extinguishing polymer grid as permanent supplemental support material in a combustible underground opening, such as within a coal mine, with which the instant invention is concerned.
While reference is made herein primarily to a combustible coal mine since the principal application of this invention will be in such environments, it is to be understood that the reference herein to a "combustible underground opening" is intended to apply more broadly to all such openings subject to the hazards of methane and dusts containing volatile matter presenting danger from explosion and fire as defined in Categories I, II, III and V of 37 C.F.R. .sctn.57.22003.